Typically in an air separation plant of the pumped or split cycle type wherein oxygen and nitrogen are generated as products, air is compressed to a pressure of about 100 psia or higher, divided into a major portion and a minor portion with the major portion being passed through reversing heat exchangers, whereby the air is at least sufficiently cooled for removing water and carbon dioxide and the minor portion, which supplies a substantial part of the required amount of cold product to balance the plant energy requirements, is compressed to a higher pressure usually from about 1200 to 3000 psia and cooled in a heat exchanger usually by the cold products obtained from the separation columns or by auxiliary refrigeration if the products are removed as liquids. If additional refrigeration is required for balancing the plant, it is generally provided by auxiliary refrigeration equipment, e.g. a Freon refrigeration unit. Other refrigeration from the minor portion can be obtained via a Joule-Thompson (isenthalpic) expansion or by work expansion (isentropic) or combination thereof prior to combining the minor portion with the major portion for forming a distillation feedstream.
It is, of course economically desirable to minimize the amount of energy required to compress the airstream and to minimize the amount of auxiliary refrigeration required to balance the refrigeration load air plant. This can be done by reducing the amount of the high pressure split or pumped portion which is compressed, cooled and expanded, and by making the refrigeration recovery process more efficient.
Cooling curves, i.e. a graph of the temperature-enthalpy relationships of the fluids to be cooled or warmed provide an indication of the efficiency of an air plant. As is known, the distance between the cooling curves is a measure of the driving force between the incoming airstream and the product stream, and is also a measure of the irreversibility of the process. As a general rule, the greater the distance between the curves the greater the energy input to achieve a balanced plant. When the cooling curves are close together, the more thermodynamically efficient the process. Hence, less energy but greater heat exchange surface is required in order to extract the refrigeration necessary to balance the plant. Thus, as the distance between the cooling curves increases, the cost of operation increases, and as the distance between the curves decreases, the capital cost increases.
In looking at cooling curves for plants producing high-pressure gases, e.g. above 600 psia, it is noted that the cooling curves tend to widen in the lower right-hand portion of the graph i.e. at a temperature below about -170.degree. F. Thus, it would be advantageous to close in the cooling curves, usually beginning at about -170.degree. F., in order to improve the thermodynamic efficiency of the plant. PG,3
Two approaches can be taken in closing the cooling curves in the lower right-hand portion of the graph, the first is by adding refrigeration at that point and the second is by removing a portion of the gas from the heat exchanger so that the remaining fluid can be cooled further. One of the basic problems with adding refrigeration at lower temperatures as shown in the lower right-hand portion of the graph is that the expense of such refrigeration is extremely high as compared to refrigeration which may be added in the upper left-hand portion. When one removes a portion of the incoming air stream midway in the heat exchanger, then there is less gas to be cooled by the product gases.
Some other problems associated with closing in of cooling curves, particularly those in air plants where liquefied oxygen and high pressure gases e.g. at pressures above 600 psig, are produced will become apparent in the description of the prior art.